Can anyone enlighten me as to the possibility that a lady named Mary could also be known as Isabella?

'My' Mary was born 1785 and is included in the Will of her brother (dated 1831). In this Will she is named "Isabella Kinnish, otherwise Clague".

I have so far been unable to reconcile the two different names as belonging to one and the same person, and any help will be appreciated.
Thank you.